The data support the conclusion that lower plants are relatively tolerant of NO2, as are higher plants. Gas-phase concentrations required to cause effects are much higher than occur in the ambient. However, caution must be exercised concerning the possible effects of nitrate and nitrite delivered to the plants via wet deposition processes as thallophytes can absorb the ions directly from the environment. Nitrite (0.5 to 1.0mM) can reduce the chlorophyll content of lichens and the photosynthetic rate of algae. Additional studies are required to determine if the wet deposition rates that occur in ambient conditions are detrimental to lower plants.